1. Field of the Invention
The present invention generally relates to implanted medical devices, and more particularly, pertains to a subcutaneous multiple electrode sensing recording and control system for an implanted cardiac pacemaker, pacemaker-cardioverter-defibrillator, drug administration device, or cardiac monitoring device.
2. Description of the Prior Art
Cardiac Arrhythmia Control Devices--Since the implantation of the first cardiac pacemaker, implantable medical device technology has advanced with the development of sophisticated, programmable cardiac pacemakers, pacemaker-cardioverter-defibrillator arrhythmia control devices and drug administration devices designed to detect arrhythmias and apply appropriate therapies. The detection and discrimination between various arrhythmic episodes in order to trigger the delivery of an appropriate therapy is of considerable current interest. Arrhythmia detection and discrimination algorithms are based in the analysis of the PQRST electrogram (EGM), usually separated for such analysis into the P-wave and R-wave, in systems which are designed to detect the depolarization of the atrium and ventricle respectively. Such systems employ detection of the occurrence of the P-wave and R-wave, analysis of the rate, regularity, and onset of variations in the rate of recurrence of the P-wave and R-wave, the morphology of the P-wave and R-wave and the direction of propagation of the depolarization represented by the P-wave and R-wave in the heart. The detection, analysis and storage of such EGM data within implanted medical devices is well known in the art.
The cardiac arrhythmias which have been treated by the aforementioned medical devices constitute variations in heart rate from the normal sinus rate range of approximately 60 to 120 beats per minute prevalent in healthy adult humans with normally functioning hearts in light to moderate exercise ranges. Bradycardia is typically characterized by rates below 60 beats per minute, although extreme bradycardia resulting in the absence of heart beat for a time sufficient to render the patient unconscious is referred to as syncope. Rates exceed about 120 beats per minute are typically characterized as tachycardia and are usually experienced as a result of such factors as physical exercise, emotional stress, pathologic cardiac disease and side effects of drugs that elevate heart rate. The normal acceleration of heart heart above 120 beats per minute in conjunction with moderate to heavy exercise is referred to as sinus tachycardia and is characterized by gradual acceleration in heart rate with normal P-wave and R-wave morphology. Tachyarrhythmias, on the other hand, are characterized by an increase in rate above 120 beats per minute not accompanied necessarily by physical exercise or emotional stress and are often accompanied by herald signs including ectopic ventricular depolarizations, unnatural morphologies and sudden onset rate increase. Such atrial or ventricular tachycardias also often spontaneously subside whereas sinus tachycardia exhibits a gradual slow down in rate with a cessation of exercise or emotional stress.
Life threatening tachyarrhythmias which require special medical treatment include high rate ventricular tachycardias and ventricular fibrillation. High rate ventricular tachycardias are characterized by fairly regular but wide morphology accompanied by some degree of hemodynamic compromise. Ventricular fibrillation is a life threatening tachyarrhythmia characterized by completely uncoordinated contractions of sections of conductive cardiac tissue of the affected chamber of the heart, resulting in a complete loss of synchronous contraction of the overall heart mass. As a consequence, the chamber ceases to pump blood effectively and in the case of ventricular fibrillation, the lack of oxygenated blood to the tissues will lead to death within minutes. Such sudden death if encountered in the hospital situation is treated by the application of cardioversion or defibrillation shock therapy. High rate atrial tachycardias and atrial fibrillation are less life threatening and patients normally recover from such episodes. However, the recurrence is suspected as a precursor to the development of life threatening ventricular arrhythmias.
In the case of the patient suffering from syncope, Stokes-Adams syndrome, sick sinus syndrome and a host of other brady/tachyarrhythmias, the preferred choice for treatment involves the implantation of a cardiac pacemaker having the capability of detecting the brady and/or tachyarrhythmia and applying pacing impulses to the heart to stimulate it to beat at a desired rate in the normal sinus range or to stimulate it at a certain high rate to treat tachycardias. In the management of tachyarrhythmia, the heart may be artificially stimulated at a faster than normal pacing rate to terminate the tachycardia or suppress premature atrial ventricular contractions which could otherwise lead to supraventricular or ventricular tachycardia, flutter or fibrillation. The pulses delivered to the heart for pacing therapy need only be of sufficient magnitude to stimulate the excitable myocardial tissue in the immediate vicinity of a pacing electrode.
More recently, the automatic pacer-cardioverter-defibrillator has been implanted in cardiac patients prone to suffer ventricular tachycardia and/or fibrillation. In such devices the functions of bradycardia and antitachycardia pacing type therapies, and cardioversion and defibrillation shock type therapies, are integrated to provide a coordinated approach to the management and treatment of ventricular arrhythmias. In such devices, therapy regimens may be programmed for the treatment of arrhythmic episodes, and the resulting successful treatment successes and failures are recorded for later analysis. In such systems, sophisticated detection algorithms for discriminating tachyarrhythmias from sinus tachycardia episodes have been proposed and implemented. The detection and discrimination of arrhythmic episode remains a subject of considerable interest inasmuch as the failure to properly discriminate may lead to the misapplication of therapies to the patient's detriment and leading to the early depletion of the implanted device's power source.
In this regard, it has also been proposed to record data upon which the detection algorithm acted in prescribing a certain therapy. Such data may include the measured interval between successive P-waves and/or R-waves, sequences of such intervals, and/or sequences of actual sampled EGMs from a point in time prior to the satisfaction of the detection algorithm to a point in time thereafter. The storage of such data has been facilitated by the implementation of microprocessor based signal processing and control systems with recirculating buffers for receiving such data and dedicated RAM into which the buffered data may be transferred. Such data may be read out at a later time by interrogation of the contents of the RAM through the use of uplink and downlink telemetry between the implanted device and an external programmer/transceiver.
It has also been proposed that implantable drug administration devices be developed as a substitute for or to augment the aforementioned brady and tachyarrhythmia control stimulation devices. In such systems, it has been proposed that antiarrhythmic drugs be delivered systemically where appropriate upon detection of the arrhythmic episodes or upon detection of other cardiac dysfunctions, such as elevated or depressed blood pressure. With the advent of chronically implantable blood-gas sensors, blood pressure sensors, mechanical activity sensors, and the like, such systems for the combined detection analysis and therapeutic treatment of various cardiac malfunctions appear to be realizable.
The aforementioned systems which depend on detection and analysis of the PQRST complex are all dependent upon the spatial orientation and number of electrodes available in or around the heart to pick up the depolarization wave front. Within the bradycardia pacing system context, it has been customary to employ pace/sense electrode pairs for detecting P-waves and R-waves and stimulating the atrium and ventricle. As described hereinafter, such pacing systems are limited in their capabilities of detecting the "capture" of the patient's heart by a pacing pulse and in detecting and storing EGM episodes of syncope, spontaneous tachycardia or pacemaker mediated tachycardia. Similarly, pacemaker-cardioverter-defibrillator arrhythmic control devices and implantable drug administration devices would benefit from enhanced capabilities of discriminating arrhythmias and storing data associated therewith.
Unipolar/Bipolar Leads--From the early days of pacing, two varieties of electrode configurations have been used for both pacing and sensing, namely unipolar and bipolar. Whether the pacemaker is called unipolar or bipolar depends on the location of both electrodes relative to the pacemaker and the heart. The unipolar and bipolar nomenclature is also applied in the pacemaker-cardioverter-defibrillator context, although bipolar and unipolar sensing may also be referred to as "near-field" and "far-field" sensing, respectively.
The unipolar electrode configuration has one pole or electrode (i.e., cathode electrode or negative pole) located on or within the heart, and the other pole (i.e., anode electrode or positive pole) remotely located from the heart. With endocardial leads, for example, the cathode is located at the distal end of the lead and typically in direct contact with the endocardial tissue to be stimulated, thus forming a "tip" electrode. Conversely, the anode is remotely located from the heart, such as comprising a portion of the metallic enclosure which surrounds the implanted device, thus forming a "can" electrode and is often referred to as the "indifferent" electrode.
The bipolar electrode configuration has both poles or electrodes typically located within the atrial or ventricular chamber of the heart. With endocardial leads, for example, the cathode is located at the distal end of the lead, again referred to as the "tip" electrode. In the bipolar configuration, the anode is usually located proximal to the "tip" electrode, spaced apart by 0.5 to 2.5 cm., and typically forming a ring-like structure, referred to as the "ring" electrode.
A variety of lead configurations can be used to pace the heart and sense its depolarizations (whether intrinsic or evoked). Atrial unipolar or bipolar electrode configurations, as well as ventricular unipolar or bipolar electrode configurations, have been used to pace the heart.
With respect to sensing, it is well known that bipolar and unipolar electrode configurations do not yield equivalent cardiac electrograms. Each configuration has advantages and disadvantages. With a unipolar sensing configuration, for example, only the electrical events adjacent to the "tip" electrode control the unipolar electrogram, while the remote "indifferent" electrode contributes negligible voltage due to its location being extracardiac.
With a bipolar sensing configuration, the magnitude of the cardiac signal will be similar for both the "ring" and "tip" electrodes, but the resulting electrogram will be highly dependent upon the orientation of the electrodes within the heart. Optimal sensing will occur, for example, when the sensing vector defined by the sensing electrodes is parallel with the dipole defined by the depolarization signal. Since bipolar electrodes are more closely spaced than their unipolar counterparts, the depolarization signal will be shorter in duration than that produced from a unipolar configuration. Due to a more restricted lead field or antenna, bipolar sensing offers improved rejection of electromagnetic and skeletal muscle artifacts and thus provides a better signal-to-noise ratio than unipolar sensing.
Post-Stimulus Residual Electrode Polarization--The delivery of an electrical stimulus to cardiac tissue induces a field which is generally orders of magnitude greater in amplitude that the field caused by the electrical activity of the tissue itself. When the stimulus ends, electrical fields remain in tissue primarily due to two factors. The first factor relates to the electrochemical equilibrium at the electrode-tissue interfaces, which has been disturbed by the stimulus, and has to reestablish itself. The second factor relates to the function of the pacemaker's output capacitor being recharged through its electrical circuits, which involve the heart as well.
When the same electrodes are used as pacing electrodes to stimulate myocardial contraction and as sensing electrodes to detect the resulting depolarization, detection of depolarization is typically somewhat reduced, because it is masked or buried in the exponential decay of the residual polarization charge on the electrode resulting from the stimulation pulse itself.
U.S. Pat. No. 4,406,286 to Stein relates to a pacemaker having an R-wave capture detection capability in which the same electrodes are utilized for both pacing and sensing (i.e., unipolar or bipolar), and wherein a bipbasic pulse is delivered for purposes of dissipating the polarization charge on the pacing electrode. The first phase is of relatively shorter duration and greater amplitude than the second phase for purposes of stimulating the myocardium, while the second phase is of relatively longer duration, lesser amplitude and opposite polarity than the first phase for purposes of providing charge compensation to neutralize the undesired electrode polarization, following which the capture detection sensing amplifier is turned on. Such "fast recharge" wave forms have been employed for many years in an attempt to facilitate short blanking and refractory time intervals following stimulation.
Limitations of Sense Amplifiers--Conventional sensing circuitry cannot be used to detect the electrogram immediately following a stimulation pulse. The relatively high output pulse, after-potentials, and electrode-tissue polarizations render the electrode blind to the induced electrogram. Since the sensing circuit gain is tuned for the relatively low voltages of the heart (i.e., 3 to 4 mV for the atrium, and 10 to 20 mV for the ventricle), the significantly greater output levels produced by the stimulation pulse (i.e., varying between 1 to 8 V) must be blocked from the sensing circuit by blanking and refractory periods so that the pacemaker is not adversely affected.
Thus, it is conventional to suppress or blank the sensing amplifier during a stimulus to avoid overloading. However, when blanking is over and the sense amplifier is reconnected, the sense amplifier may abruptly sense a different potential than was present at the time of initial blanking, due to the after-potentials and electrode polarization as well as the recharge function, all of which can produce unwanted artifacts in the sensing signal.
"Capture" Defined--Capture is defined as an evoked cardiac response to a pacemaker output or stimulation pulse. In a pacemaker with dual-chamber pacing capabilities, for example, a stimulation pulse can be applied to either the heart's atrium or the ventricle during an appropriate portion of a cardiac cycle. The minimum output pulse energy which is required to capture and thus evoke a muscular depolarization within the heart is referred to as the stimulation threshold, and generally varies in accordance with the well known strength-duration curves, wherein the amplitude of a stimulation threshold current pulse and its duration are inversely proportional.
A number of factors can influence changes in the stimulation threshold for each patient, however, following implantation of the pacemaker and pacing lead. Factors which can influence both acute and chronic stimulation thresholds include, for example: (1) changes in position of the pacing electrode relative to the cardiac tissue; (2) long-term biologic changes in cardiac tissue closely adjacent the electrode, such as due to fibrotic tissue ingrowth; (3) changes in the patient's sensitivity to stimulation as a function of periodically fluctuating conditions, even on a daily basis, due to various causes such as diet, exercise, administered drugs, electrolyte changes, etc.; and (4) gradual changes in pacemaker/lead performance due to various causes such as battery depletion, component aging, etc.
Capture Detection and Adjustable Output Pulse Energy--To conserve battery power and extend the pacemaker's useful life, it is usually desired to achieve capture at the lowest possible energy setting for the output pulse. With the advancement of programmable pacemakers, it became common to initially program an output pulse energy setting which includes a safety margin somewhat above that required to produce capture. These programmable pacemakers include a programmable output stimulation pulse which permits the physician to select an output pulse energy which is known to be sufficient to capture the heart but which is below the maximum obtainable output energy of the pacemaker. Such output pulse energy adjustments are usually accomplished by the attending physician during an office visit with the use of an external programmer and an electrocardiogram (ECG) monitor. At this time, the physician may assess capture by means of an ECG measured through ECG electrodes placed on the patient's limbs and/or chest, during which time the pacemaker is providing a sequence of temporarily programmed-in stimulation pulses with decreasing pulse energies in a system of the type described in U.S. Pat. No. 4,250,884 to Hartlaub, et al. For example, capture detection of the ventricle is confirmed by the presence of the evoked QRS complex or R-wave, and capture detection of the atrium is confirmed by the presence of the evoked P-wave. Loss of capture can be directly observed and correlated to the pulse energy at which capture is lost.
Since the late 1960's, self-adaptive pacemakers have been developed which have the capability of automatically adjusting the energy content of the pacing pulse as appropriate to accommodate changes in stimulation threshold.
U.S. Pat. No. 3,757,792 to Mulier et al, for example, relates to an early pacemaker which provides for a decreased battery drain by sensing each driven heart beat (i.e., R-wave) and providing for a decrease in energy for each succeeding output pulse until such time as loss of capture is detected. Following a detected loss of capture, the next succeeding output pulse is increased in energy by an amount to be safely over the threshold hysteresis level. U.S. Pat. No. 3,949,758 to Jirak (incorporated herein by reference) relates to a similar threshold-seeking pacemaker with automatically adjusted energy levels for output pulses in response to detected loss of capture (i.e., absence of R-wave), and describes separate sensing and pacing electrodes, which are each utilized in unipolar fashion with a third common electrode having a comparatively larger dimension, to reduce residual polarization problems.
U.S. Pat. No. 3,977,411 to Hughes, Jr. et al shows a pacemaker having separate sensing and pacing electrodes which are each utilized in unipolar fashion. The sensing electrode comprises a ring electrode having a relatively large surface area (i.e., between 75 to 200 mm.sup.2) for improved sensing of cardiac activity (R-waves), and is spaced along the pacing lead approximately 5 to 50 mm from the distally-located tip electrode used for pacing.
U.S. Pat. No. 3,920,024 to Bowers shows a pacemaker having a threshold tracking capability which dynamically measures the stimulation threshold by monitoring the presence or absence of an evoked response (R-wave). If no R-wave is sensed within a post- stimulus interval (e.g., 20 to 30 ms post-stimulus), the pacemaker delivers a closely-spaced backup pulse (e.g., 40 to 50 ms post-stimulus) at increased amplitude and pulse width to ensure an evoked response. Various electrode configurations are illustrated in FIGS. 1B and 9A-9F for purposes of sensing, including those of sensing with an endocardial lead extending into the right ventricle, wherein in one embodiment the sensing is between one intracardiac electrode and a reference electrode which is spaced some distance away from the heart, and in another embodiment the sensing is between intracardiac electrodes.
U.S. Pat. No. 4,305,396 to Wittkampf et al (incorporated herein by reference) also relates to a rate-adaptive pacemaker in which the output energy is automatically varied in response to the detection or non-detection of an evoked response (R-wave) and the detected stimulation threshold. For the stated purpose of facilitating prompt post-stimulus R-wave sensing, the pacemaker delivers a two-portion output, wherein the first portion comprises a positive-going recharge pulse for compensation of the repolarization caused by the stimulus pulse, and wherein the second portion comprises a negative-going stimulus pulse. Similar to the above mentioned Bowers patent, the pacemaker delivers a backup pulse within a post-stimulus interval of time (e.g., 50 to 100 ms post-stimulus) at an increased amplitude, such as twice the amplitude of the previously-delivered stimulus pulse if the applied stimulus fails to capture the heart. It is stated to be preferred to use the same electrode for both pacing and sensing, such as a unipolar or bipolar system wherein there is at least one electrode located in the ventricle, but suggests that other lead designs may be utilized such that the sensing and pacing electrode are separate.
U.S. Pat. No. 4,387,717 to Brownlee et al relates to a pacemaker having a separate (i.e., non-pacing) electrode element, implanted near or in direct contact with the cardiac tissue, and positioned relative to the pacing electrodes (i.e., unipolar pacing from "tip" to "can") to provide improved P-wave and R-wave sensing with minimal interference from the pacing electrodes. The "can" functions as an indifferent electrode for sensing in combination with the separate electrode element. The separate sensing electrode is spaced from the pacing electrodes to minimize cross coupling and interference from the pacing stimulus and after-potentials. The separate sensing electrode comprises an extravascular metallic plate having a comparatively large surface area in one embodiment. In another embodiment the separate sensing electrode comprises a cylindrical metal ring mounted on the insulated pacing lead between the pacemaker and the "tip" electrode, and is described as being located along the lead to permit positioning the sensing electrode either within the heart, externally on the heart wall, or in some remote location in the vascular system away from the heart.
U.S. Pat. No. 4,585,004 to Brownlee relates to an implantable cardiac pacemaker and monitoring system, wherein the pacing-sensing electrode system is electrically separate from an auxiliary sensing system. The auxiliary sensing system comprises a transvenous data lead with ring electrodes for sensing located in the right ventricle (approximately 1 cm from the pacing tip electrode for R-wave sensing) and in the right atrium (approximately 13 cm from the tip electrode to be in close proximity with the S-A node), both ring electrodes being used in conjunction with the pacemaker can in unipolar sensing fashion.
U.S. Pat. No. 4,686,988 to Sholder relates to a dual chamber pacemaker having atrial and ventricular endocardial leads with a separate proximal ring electrode coupled to a P-wave or R-wave sensing EGM amplifier for detecting the atrial or ventricular evoked response to atrial or ventricular stimulation pulses generated and applied to other electrodes on the endocardial lead system. The auxiliary lead system thus resembles the Brownlee '004 patent.
U.S. Pat. Nos. 4,759,366 and 4,858,610 to Callaghan, et al, incorporated herein by reference, relate to evoked response detector circuits which also employ fast recharge in at least one separate sensing electrode in either unipolar or bipolar electrode configurations in either or both the atrium and ventricle. The cardiac pacing systems function as unipolar and bipolar systems at different steps in the operating cycle. In the '610 patent, a separate electrode on the connector block of the pacemaker can is suggested for use as the reference electrode anode rather than the metal case itself if the case is employed as the reference electrode for the delivery of the stimulation pulse. In the '366 patent, the detected evoked response is used in an algorithm for adjusting the pacing rate.
U.S. Pat. No. 4,310,000 to Lindemans and U.S. Pat. Nos. 4,729,376 and 4,674,508 to DeCote, incorporated herein by reference, also disclose the use of a separate passive sensing reference electrode mounted on the pacemaker connector block or otherwise insulated from the pacemaker case in order to provide a sensing reference electrode which is not part of the stimulation reference electrode and thus does not have residual after-potentials at its surface following delivery of a stimulation pulse.
The DeCote '376 and '508 patents also set forth stimulation threshold testing algorithms for adjusting the pacing pulse energy.
Thus, considerable effort has been expended in providing electrode systems, fast recharge circuitry and separate sense amplifier systems for avoiding after-potentials and providing capture detection and stimulation threshold tracking.
Data Recording Systems--Turning to EGM data recording systems, heart rate, interval and morphology recording has been suggested in U.S. Pat. Nos. 4,003,379 and 4,146,029 to Ellinwood, Jr., and subsequently in U.S. Pat. No. 4,223,678 to Langer, et al and U.S. Pat. No. 4,295,474 to Fischell, et al. Such implantable recording systems have employed bipolar or unipolar electrode systems of the type described above in the recording of near-field or far-field EGM data. Thus the quality of EGM data recorded is limited by the limited electrode pathways and possible vectors.
Distinguishing Arrhythmias--Distinguishing malignant tachyarrhythmias from sinus tachycardias and detecting pacemaker mediated tachycardias is similarly limited by the available electrode configurations employed in single and dual chamber pacing systems, implantable drug dispensers and pacemaker-cardioverter-defibrillator systems as described above. In the context of discriminating spontaneously occurring tachyarrhythmias from sinus tachycardia, attempts have been made to employ both atrial and ventricular electrode systems in order to determine whether the tachycardia is sinus in origin or reflects a retrograde conducted abnormal ventricular rhythm. For example, it is known to have placed multiple electrodes on atrial and ventricular leads and to sense the direction of travel of a depolarization wave front as shown for example in U.S. Pat. No. 4,712,554 to Garson, Jr.
In addition, it has been found that pacemakers which operated in the DDD or related modes can, under certain circumstances, sustain a dangerous tachycardia condition particularly when operating at an upper rate limit. A pacemaker sustained or mediated tachycardia (PMT) condition is defined as an operational pacing state wherein the pacemaker erroneously stimulates the ventricle of a heart at the pacing upper rate limit for sustained periods of time. Such PMT behavior is initiated when a ventricular event occurs at a time during which the myocardial tissue between the atrium and ventricle can transmit retrograde electrical signals from the ventricle to the atrium which in turn cause an atrial depolarization. The sensing of the resulting atrial depolarization by the atrial sense amplifier in turn causes the ventricular pulse generator to emit a ventricular pacing pulse after the AV time period times out. The cycle may repeat itself if the ventricular pace event is conducted to the atrium where it again causes an atrial depolarization which is picked up by the atrial sense amplifier. This repetitive high rate stimulation may be sustained indefinitely by the pacemaker causing discomfort to the patient or possibly inducing more threatening arrhythmias.
Various techniques have been implemented to minimize th impact of PMTs, but these techniques usually sacrifice flexibility of the DDD system. U.S. Pat. No. 4,967,746 to Vandegriff sets forth a number of techniques which have been employed to alleviate PMTs.
ECG/EGM Vector Analysis--The aforementioned Lindemans U.S. Pat. No. 4,310,000 suggests various modifications to the passive sensing reference electrode depicted in its drawings, including the incorporation of more than one passive sensing reference electrode provided on or adjacent to the metallic can, positioned as deemed necessary for best sensing, and connected to one or more sense amplifiers. No specific use of the additional passive sensing reference electrodes is suggested, although the single passive sensing reference electrode is suggested for use with a sense amplifier to detect both capture and spontaneous atrial or ventricular electrical events in a dual chamber pacing system.
It is known in the art to monitor electrical activity of the human heart for diagnostic and related medical purposes. U.S. Pat. No. 4,023,565 issued to Ohlsson describes circuitry for recording EKG signals from multiple lead inputs. Similarly, U.S. Pat. No. 4,263,919 issued to Levin, U.S. Pat. No. 4,170,227 issued to Feldman, et al, and U.S. Pat. No. 4,593,702 issued to Kepski, et al, describe multiple electrode systems which combine surface EKG signals for artifact rejection.
The primary use for multiple electrode systems in the prior art appears to be vector cardiography from EKG signals taken from multiple chest and limb electrodes. This is a technique whereby the direction of depolarization of the heart is monitored, as well as the amplitude. U.S. Pat. No. 4,121,576 issued to Greensite discusses such a system.
In addition, U.S. Pat. No. 4,136,690 issued to Anderson, et al, shows a vector cardiographic system used for arrhythmia analysis. Similar techniques are described in "Rhythm Analysis Using Vector Cardiograms," Transactions on Biomedical Engineering, Vol. BME-32, No. 2, Feb. 1985, by Reddy, et al, European Pat. No. 0 086 429 issued to Sanz and U.S. Pat. No. 4,216,780 issued to Rubel, et al.
Various systems have additionally been proposed for measuring the orthogonal ventricular or atrial electrogram from multi-electrode lead systems placed endocardially within the patient's atrium and/or ventricle. Such orthogonal endocardial EGM systems are depicted in U.S. Pat. No. 4,365,639, issued to Goldreyer, and U.S. Pat. Nos. 4,630,611 and 4,754,753 issued to King. In addition, orthogonal ventricular electrogram sensing employing endocardial, multi-electrode lead systems and associated circuitry are disclosed in two articles by Goldreyer, et al, entitled "Orthogonal Electrogram Sensing," PACE, Vol. 6, pp. 464-469, March-April 1983, Part II, and "Orthogonal Ventricular Electrogram Sensing," PACE, Vol. 6, pp. 761-768, July-August 1983. In the Goldreyer patent and in these papers, it is suggested that the orthogonal electrodes be employed to detect paced events and provide capture verification as well as to facilitate the discrimination of P-waves from QRS complexes. Other articles by Goldreyer, et al., appear in the literature, including those listed in the bibliographies to these two papers.
The aforementioned King U.S. Pat. Nos. 4,630,611 and 4,754,753 describe X, Y and Z orthogonally displaced electrodes on the body of the endocardial pacing lead and circuitry for developing a composite EGM vector signal in order to detect changes in the vector over time and discriminate normal sinus rhythm from tachyarrhythmias.
Finally, U.S. patent application Ser. No. 611,901 entitled "Multi-Vector Pacing Artifact Detector," filed Nov. 9, 1990, and assigned to the assignee of the present application, sets forth a system for detecting the artificial pacing artifact in patients having artificially paced myocardial contractions in an external monitor employing three standard EKG leads with chest or limb electrodes.
Rate Adaptive Pacing--As described in the aforementioned Callaghan '610 patent, the use of physiologic parameters to develop a control signal for adapting the pacing rate to physiologic requirements has become an important aspect of current pacing systems. The stimulus-repolarization T-wave interval (Q-T interval) has been used in Vitatron.RTM. pacemakers described in U.S. Pat. No. 4,228,803 to Rickards.
ECG/EKG Electrode Systems--numerous body surface ECG monitoring electrode systems have been employed in the past in detecting the ECG and conducting vector cardiographic studies. For example, U.S. Pat. No. 4,082,086 to Page, et al., discloses a four electrode orthogonal array which may be applied to the patient's skin both for convenience and to ensure the precise orientation of one electrode to the other. U.S. Pat. No. 3,983,867 to Case describes a vector cardiography system employing ECG electrodes disposed on the patient in normal locations and a hex axial reference system orthogonal display for displaying ECG signals of voltage versus time generated across sampled bipolar electrode pairs.
Finally, in regard to subcutaneously implanted EGM electrodes, the aforementioned Lindemans U.S. Pat. No. 4,310,000 discloses one or more reference sensing electrode positioned on the surface of the pacemaker case as described hereinbefore. U.S. Pat. No. 4,313,443 issued to Lund describes a subcutaneously implanted electrode or electrodes for use in monitoring the ECG.